This invention relates to a method of producing mask-blanks (may be referred to as “blanks” or “mask blank” or “mask blanks” or “photomask blank” or “photomask blanks” or “photomask blanks substrate”) including a resist coating process of uniformly applying a resist solution and uniformly forming a resist film on a square-like (a square or a rectangular) substrate. This invention also relates to the blanks having the resist film formed on the square-like substrate.
In order to produce the blanks by forming a resist film on a square-like substrate with or without a single-layer or a multi-layer thin film of various types deposited thereon, use is generally made of a spin-coating method using a resist coating apparatus for coating the substrate with the resist film by applying and dispensing a resist solution onto the substrate and rotating the substrate. For example, Japanese Patent Publication (JP-B) No. H4-29215 (corresp. to U.S. Pat. No. 4,748,053) disclosed a resist spin-coating method capable of forming a uniform resist film which is not thickened at four corners of a square-like substrate.
The spin-coating method disclosed in the above-mentioned publication comprises a uniformly coating step and a drying step. In the uniformly coating step, a resist solution is dispensed onto a substrate, and the substrate is rotated at a preselected rotation speed for a preselected rotation time to form a resist film having a uniform thickness within the substrate including four corners of the substrate. The preselected rotation speed and the preselected rotation time are determined so that a product of the preselected rotation speed and the preselected rotation time is less than 24000 (rpm·sec) with maintaining the preselected rotation time less than 20 seconds. In the drying step, the substrate is rotated at a rotation speed lower than the preselected rotation speed in the uniformly coating step to dry the resist film while keeping the resist film uniformity obtained in the uniformly coating step.
In the above-mentioned publication, description is made of a specific embodiment using a chromium-coated substrate (127 mm×127 mm) and a resist solution which contains a main-chain-fragmentation resist comprising a high-molecular-weight resin and solvent and which typically has a relatively high viscosity, such as poly(butene-1-sulfone) having a viscosity of 30 cp or poly(glycidyl methacrylate) having a viscosity of 15 cp. The resist solution is applied and dispensed onto the substrate, and the substrate is rotated under a rotating condition previously selected within the above-mentioned range. Then, the substrate is subjected to a predetermined heating/drying (drying-by-heating) process. Thus, a resist film is formed. Non-uniformity in thickness of the resist film is suppressed below 90 Å and further to 50 Å in an effective pattern forming region (critical area) of 107 mm×107 mm at the center of the substrate where an effective pattern (main pattern or device pattern) to be transferred to an object will be formed.
In recent years, a reduction projection exposure apparatus of a step-and-repeat system (stepper) was adapted to expose a larger exposure area (field). Further, a reduction projection exposure apparatus of a step-and-scan system (scanner) was developed. As a consequence, a transfer mask or reticle (mask, hereafter) and the blanks as a material thereof are increased in substrate size from 127 mm×127 mm to 152.4 mm×152.4 mm. Following the increase in substrate size, the critical area on the mask and the blanks is also enlarged to 132 mm×132 mm. Furthermore, a long side of the critical area (in parallel to a scanning direction of the scanner) is increased in length to 140 mm.
Moreover, there is a recent demand for improvement in CD (Critical Dimension) uniformity (dimensional accuracy) following miniaturization of a pattern to be transferred to a semiconductor substrate or the like by the use of the mask produced from the blanks.
Following the above-mentioned demand, a requirement for the resist film thickness uniformity within a substrate becomes more and more strict. In the critical area mentioned above, the resist film thickness uniformity within a substrate (i.e., the difference between the maximum thickness and the minimum thickness of the resist film within the critical area) is requested to be 100 Å or less, more preferably 50 Å or less.
In addition to the main pattern in the critical area, the mask is also provided with auxiliary patterns, such as an alignment mark, a barcode, and a quality assurance pattern, formed in an outer peripheral portion surrounding the critical area at the center of the mask or the blanks substrate. Since the critical area is enlarged, these auxiliary patterns are formed in an area closely adjacent to the outer peripheral end (the edge) of a principal surface of the substrate.
Further, following the miniaturization of the pattern, a resist material was also changed. For example as a positive resist, use has generally been made of a main-chain-fragmentation resist comprising a high-molecular-weight resin or a dissolution-inhibited resist comprising a novolak resin and a dissolution inhibitor. However, those resists are recently replaced by a chemically amplified resist comprising, for example, a poly(hydroxystyrene) (PHS) resin and a photo acid generator, and assuring a higher resolution and a higher sensitivity.
Furthermore, a part of the resist film formed at a peripheral end portion of the substrate (edge bead) may be stripped off or flaking during handling of the substrate, and may cause dust (particle defect) which may not only result in a product defect but also may prevent the substrate from being accurately handled or held in subsequent processes and steps. In view of the above, it is requested to remove an unnecessary part of the resist film formed at the peripheral end portion (i.e. edge bead) of the blanks substrate. For the mask production, various resist species are proposed till now. As a concourse, it becomes difficult to suppress thickness non-uniformity of the resist film in the critical area to 100 Å or less, further 50 Å or less by the conventional resist spin-coating method which has been used in the blanks production, as a result of enlargement of the blanks substrate size and the critical area mentioned above.
In particular, the chemically amplified resist, which has recently been drawing attention as a resist material and which comprises, for example, a poly(hydroxystyrene) (PHS) resin and a photo acid generator, is dissolved by a solvent to obtain a resist solution. The solvent generally consists of or contains, as a main component, one of propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monomethyl ether (PGME), methyl isoamyl ketone (MAK), and ethyl lactate (EL). The above-mentioned resist solution has an average molecular weight less than 100,000 and generally has a low viscosity (less than 10 mPa·s) and is easily dryable. When the conventional spin-coating method is carried out, the resist film is at first uniformly formed on a square-like substrate in the uniformly coating step. However, the resist solution accumulated in the outer peripheral portion of the substrate (in particular, at four corners of the substrate) tends to be drawn back towards the center of the substrate during rotation of the substrate in the uniformly coating step, as well in the drying step following the uniformly coating step, and tends to be dried while being drawn back.
Under the above-mentioned situation where the critical area is enlarged (for example, an area of 132 mm×132 mm at the center of the substrate), desirable resist film thickness uniformity of 100 Å or less or 50 Å or less within the critical area, can hardly be obtained.
As described above, the auxiliary patterns are formed in the area closely adjacent to the outer peripheral end (i.e. edge) of the substrate, as a result of enlargement of the critical area. According to the conventional resist spin-coating method, the resist film thickness becomes extremely large or sometimes extremely small in the outer region where the auxiliary patterns are formed. In this event, the auxiliary patterns can not be formed in exact conformity with a designed dimension or a desired fidelity in shape, and a pattern error can be caused.
Moreover, the chemically amplified resist comprising, for example, the poly(hydroxystyrene) resin and the photo acid generator is generally brittle when the resist film is formed by the spin-coating and the subsequent heating and drying (i.e. baking), as compared with a main-chain-fragmentation resist or a crosslinking resist comprising a high-molecular weight resin and having been used so far. The resist film is stripped off or flaking by contact with a storage container, a delivery container, or various other processing apparatuses, causing dust (particle defect), in case when the resist film formed at the peripheral end portion of the substrate (i.e. edge bead) is not removed. This results in an increase in occurrence of defects of the mask or the blanks (product).